High-strength low-alloy steels having improved formability

ABSTRACT

FULLY KILLED HIGH-STRENGTH LOW-ALLOY STEELS CONSISTING ESSENTIALLY OF .06% TO .20% CARBON, .50% TO 1.4% MANGANESE, .01% TO .08% COLUMBIUM OR .04% TO .12% VANADIUM, .05% MAXIMUM SILICON, .04% MAXIMUM SULFUR, .04% MAXIMUM PHOSPHORUS AND AN INCLUSION SHAPE-CONTROL AGENT COMPRISING EITHER .06% TO .20% ZIRCONIUM, .01% TO .10% OF A REARE EARTH OR.01% TO .10% MISCHMETAL ARE CHARACTERIZED IN A HOT-ROLLED FINISHED CONDITION BY YIELD STRENGTHS IN EXCESS OF 45,000 P.S.I., ULTIMATE TENSILE STRENGTHS IN EXCESS OF 60,000 P.S.I., DUCTILITIES AS MEASURED BY PERCENT ELONGATION (2 INCHES) IN EXCESS OF 20%, GOOD TOUGHNESS, SUPERIOR FORMABILITY AND REDUCED DIRECTIONALITY . THE STEELS ARE HOT-ROLLED FINISHED IN THE TEMPERATURE RANGE 1550*F. TO 1650*F., COOLED AT A RATEA WITHIN THE RANGE 20*F. TO 135*F. PER SECOND AND COLLECTED BY COILING OR PILING WITHIN A TEMPERATURE RANGE OF 1025*F. TO 1175*F.

1972 I M. KORCHYNSKY L 5 5,570

HIGH-STRENGTH LOW-ALLOY STEELS HAVING IMPROVED FORIIABILITY Filed July1e, 1969. z Sheets-Sheetfl mvamoks MICHAEL KORCHYNSKY I JOHN DAVIDGROZIBR JOHN L. MIHELICH My 30, 1972- 3,666,570 amn swnmiwe'ru LOW-ALLOYSTEELE HAVING IMPROVED FORMABILIIY Filed July 16 1969 M. KORCHYNSK Y- ETAL 2 Sheets-Sheet z 8 mm. F

INVENTORS MICHAEL KORCHYNSKY JOHN DAVID GROZIER JOHN L. MIHELICH UnitedStates Patent 3,666,570 HIGH-STRENGTH LOW-ALLOY STEELS HAVING IMPROVEDFORMABILITY Michael Korchynsky, John David Grozier, and John L.

Mihelich, Bethel Park, Pa., assignors to Jones & Laughlin SteelCorporation, Pittsburgh, Pa. Filed July 16, 1969, Ser. No. 842,407

Int. Cl. C22c 39/54 s. Cl. 148-36 9 Claims ABSTRACT OF THE DISCLOSUREFully killed high-strength low-alloy steels consisting essentially of.06% to .20% carbon, .50% to 1.4% manganese, .01% to .08% columbium or.04% to .12% vanadium, .05% maximum silicon, .04% maximum sulfur, .04%maximum phosphorus and an inclusion shape-control agent comprisingeither.06% to .20% zirconium, .01% to .10% of a rare earth or .01% to .10%mischmetal are characterized in a hot-rolled finished condition by yieldstrengths in excess of 45,000 p.s.i., ultimate tensile strengths inexcess of 60,000 p.s.i., ductilities as measured by percent elongation(2 inches) in excess of 20%, good toughness, superior formability andreduced directionality.,The steels are hot-rolled finished in thetemperature range 1550" F. to l*650 [F., cooled at a rate within therange 20 F. to 135 -F. per second and collected by coiling or pilingwithin a temperature range of 1025 F. to 1175 F.

This invention relates to high-strength low-alloy steels characterizedby a desirable balance and uniformity of physical properties anddistinguished by their formability and reduced directionality.

- We have developed a class of high-strength low-alloy steels which in ahot-rolled finished condition exhibit good toughness, ductility andstrength. In addition, the steels are of superior formability andreduced directionality, that is, the longitudinal (parallel to therolling direction) and transverse (across the rolling direction)properties of'the steels, with respect to notch toughness and ductility,are more nearly the same. The improved formability and reduceddirectionality are brought about through the use of an inclusionshape-control agent comprising either zirconium, a rare earth, ormischmetal which, of course, is a mixture of rare earths. The use of aninclusion shape-control agent results in the formation of substantiallyspherically-shaped inclusions which retain their spherical shape in thefinished material. This inclusion morphology results in a reduction ofthe directionality of the steels by improving their resistance toductile fracture in the transverse direction and by making theirlongitudinal and transverse ductilities more nearly alike. In addition,the formability of the steels is improved.

The steels of the invention employ either vanadium or columbium as astrengthening agent and are processed within definite finishing andcollecting temperature ranges to produce desired properties in the steeldirectly oil? the hot-mill.

Accordingly, an object of the present invention is to provide low-alloysteels having high strength in combination with goodtoughness andductility, superior formability and reduced directionality. Anotherobject of the invention is to provide such steels characterized in ahot-rolled finished condition byyield strengths in excess of 45,000p.s.i., ultimate tensile strengths in excess of 60,000 p.s.i.,ductilities as measured by percent elongation (2 inches) in excess of20% and good toughness. Still another object of the invention is toprovide such steels having improved resistance to ductile fracture inthe transverse direction.

These and other objects and advantages of the present invention willbecome apparent from the following detailed description thereof withreference to the drawings wherein FIGS. 1 through 10 are photographicreproductions of steel specimens which have been subjected to bendingtests and illustrate the improved formability of the steels of theinvention.

The steels of the present invention are fully killed and have thefollowing general chemistry: carbon, .06% to .20%; manganese, .50% to1.4%; columbium, .01% to .08% or vanadium, .04% to .12% silicon, .5maximum; sulfur, .04% maximum; phosphorus, 04% maximum; an inclusionshape-control agent comprising either 06% to .02% zirconium, .01% to.10% of a rare earth, or .0l% to 10% mischmetal; and balance iron.

The preferred steels of the invention consist essentially of .10% to.15% carbon, .9% to 1.2% manganese, .02% to .04% columbium or .04% to.07% vanadium, .05% maximum silicon, .025% maximum sulfur, .03% maximumphosphorus, .08% to .12% zirconium or .01% to .10% of a rare earth ormischmetal, balance iron. Rare earths which are employed in the steelsof the invention are, for example, cerium, lanthanum, praseodymium,neodymium, yttrium and scandium.

The steels to possess the desired characteristics and properties of ayield strength in excess of 45,000 p.s.i., an ultimate tensile strengthin excess of 60,000 p.s.i., ductility as measured by percent elongation(2 inches) in excess of 20% and a superior toughness are hot-rolledfinished in the temperature range of 1550" F. to 1650" F. and collectedby coiling or piling within a temperature range of 1025 F. to 117 5 F.For the typical length of a modern hot-mill run-out table andconventional rolling speeds, the steel must be cooled at a rate within arange of 20 F. to 135 F. per second to maintain finishing and coilingtemperatures within these ranges. Steels finished and/or collected attemperatures in excess of the temperatures set out above generallyexhibit strengths below a yield strength of 45,000 p.s.i. and anultimate tensile strength of 60,000 p.s.i. In addition, the steels donot have as good impact properties as steels hot rolled within thetemperature ranges set out above. Steels finished or coiled below thedesired temperature ranges exhibit ductilities as measured by percentelongation inferior to the ductilities of steels of the invention. Inaddition, low finishing temperatures result in production liabilities inthat rolling speeds must be slower to achieve the lower finishingtemperatures.

As noted above, the inclusion shape-control agents cause the sulfideinclusions in the steel to retain a spherical form, resulting in asignificant improvement in the formability of the material and reducingthe directionality of the steels. In the absence of an inclusionshapecontrol agent, the inclusions become elongated during hot rollingand aligned parallel to the rolling direction and contribute to thedifferences in ductility and impact energy absorbed ductile fracture)bet-ween longitudinal and transverse test sections of the steels.

Suflicient zirconium is added to the steels of the invention so thatthere is a minimum of .02% zirconium in the steel in excess of thezirconium which combines with the nitrogen in the steel to formnitrides. For a typical highstrength low-alloy steel containing .006%nitrogen, therefore, approximately a minimum of .06% zirconium is iceadded to the steel. The minimum amount of zirconium re-- quired is givenby the following formula: percent zirconium=0.02% zirconium+6.5 (wt.percent N). The zirconium is preferably added to the steel in the ingotmold during teeming. Zirconium additions are made when the mold is aboutone-third full and the additions completed by the time the moldis abouttwo-thirds full. Typical zirconium recoveries achieved employing thismethod of addition are about 60%. The zirconium additions can also bemade to the ladle after the heat is tapped. However, the steel in theladle must be first fully killed toassure good zircon.- ium reCoveryQInthis techniquefit is important to employ goodteeming practice tominimize oxygen or nitrogen entrainment during teeming which adverselyaffects zirconium recovery.

. .The reduced directionality of the steels of the invention withrespect to increasedtransverse impact shelf energies andmore nearlyalike transverse and longitudinal ductilities is shown'in the table. Allof the steels listed in the table were hot-rolled finished within 1550"F. to 1650 F. and collected within 1025 F. to l175 1 While Steel 1contained .0l% zirconium it is considered to have not been treated withzirconium since that amount of zirconium is insuflicient to bring aboutthe desired inclusion morphology. The specimens for which the data ofthe table were obtained comprised one-half size Charpy V- notch samples,except for Steel 5 where one-third size samples were employed. Theimpact energies set out in the table are at 100% ductile fracture of thespecimens.

rare earths or' mischm'etalare used as the inclusion shape control.agent, additional carbon is not needed to maintain a given strengthlevel.

We claim: I

1. A kill high-strength low-alloy steel which has been hot-rolledfinished in the temperature range 1550 F. to 1650" E, cooled at a ratewithin the :r-ange bfZOiLIE. to 135 F. per second, and collected withina temperature range of 1025 F. to 1175 F., the steel beingcharacten izedin a hot rolled conditionby a yield strength inlexc'ess of 45,000p.s.i., an ultimatetensile strength in excess of 60,000 p.s.i.,ductility as measured by percent elongation (2 inches) in excess of 20%,good toughness andjformability and reduced directionality, said steelconsisting essentially of 06% to 20% carbon, .50% to 1.4% manganese, astrengthening agent 'selectedfroin the group consisting of .0l% to .08%columbiumand 04%" to .12% vanadium, .5 maximum silicon, sulfur in anamount up to 04%, .04% maximum phosphorus, asulfide inclusionshape-control agent selected .from the group consisting of 06% to 20%zirconium, .01%-to .10%"of a rare earth and .01% to .10% mischmetal,balance iron,

TABLE Chemistry (wt. percent) Yield iiiisii .Pereent Impact Steel 0 MnSi Al V Cb Zr N Treatment ir ehtlon ii ii ii i i 2 i I l ti l b s 1 .10.00 .052 .075 .047 .01 .007 None.- .-{gg gggg 22g Zifigg 33 2 .11 1.07.047 .073 .05 .10 .007 Zr 32g g ggg 538 @3 8 12g 3 J2 m8 -----{%2E$t32i13% 253%88 $53533 4 Zr "0 32553532033: 1528 333.133 351533 3323 ii isas? 20,233 2; a J04 Zr --i%?i$t32i..:: at an an: at "a 27 .071 .030 .087.007 z: :ggg gfigg 338 gig;-

m i a: as: as; s

.021 .001 --.042 .11 .006 Zl' 74,000 igg 3 g The improved formability ofthe steels of the invention is shown by FIGS. 1 through 10 of thedrawings. Samples were sheared from Steels 1 through 10 of the table andsubjected to a 90 bend. The inside bend radius for all specimens exceptthe specimens of Steel 5 was .250 inch. The inside bend radius for thespecimens of Steel 5 was .125 inch. FIGS. 1 through 10 representspecimens taken from Steels 1 through 10 of the table, respectively. Asshown in the drawings, the steels which did not contain zirconium, FIGS.1, 3, 5, 7 and 9, cracked upon bending. Of the specimens from the steelscontaining zirconium, FIGS. 2, 4, 6, -8 and 10, only specimens fromSteels 6 and 8 exhibited cracking, but to a very minor degree andsubstantially less than the specimens of the steels having approximatelythesame chemistry but not containing zirconium. i 7

Equivalent reduced'directionality and-improved formability is obtainedusing rare earths and mischmetal rather thanzirconium" as the inclusionshape-control agent. In this regard, we have-found-that in order tomaintain a given strength.- level for the steels of the inventioncontaining vanadiumand employing zirconium as the inclusion shapecontrol agent it is necessary to increase the carbon content of thesteel. This is because the strength of the steel is derivedfro'm'vanadium nitride precipitates and when-zirconium is added thenitrogen preferentially combines with the zirconium andstrengthening bythe formation of vanadium nitrides does not occur. However, when thesulfide inclusions in the steel having a substantially spherical shape.I: 2. The steel of claim 1 wherein the strengthening v agent comprises01% to .08% columbium.

3. The steel of claim 1 wherein the strengthening agent comprises .04%to .12% vanadium and the sulfide inclusion shape-control agent comprises01% to .10% of a rare earth. i 4. A killed high-strength low-alloy steelwhich has been hot-rolled finished in the temperaturerange l550"F.' to1650" F., cooled at a rate within the range-of 20 F. to F. per secondand collected Within a temperature range of 1025 F. to 1175 F. thesteelbeing charaeterized in a hot rolled condition by a yield strengthin excess of 45,000 p.s.i., an ultimate tensile strengthin' excess; of60,000 p.s.i., ductility as measured by' percent elongation (2 inches)in excess of 20%, good toughness and form'ability and reduceddirectionality, said stee'l consisting essentially of .10% to .15%carbon, .9% to 1.2% manganese, a strengthening agent selected from thegroup consisting of: .02% to .04% columbium and --.04-%' to .07%vanadium, 05% maximum' silicon, sulfur in an amount up to .025 %,'.03maximum phosphorus, a sulfide inclusion shape-control agent-selectedfromthe group consisting of -.08% to".12% zirconium, .0l% to -.10% of a rareearth and 01% to .10% mischmetaljbala'nce iron, the sulfide inclusionsin the steel having a substantially spherical shape.

5. The steel of claim 4 wherein the strengthening agent comprises .02%to .04% columbium.

6. The steel of claim 5 wherein the sulfide inclusion shape-controlagent comprises .08% to .12% zirconium.

7. The steel of claim 5 wherein the sulfide inclusion shape-controlagent comprises .01% to .10% of a rare earth.

8. The steel of claim 4 wherein the strengthening agent comprises .04%to .07% vanadium.

9. The steel of claim 8 wherein the sulfide inclusion shape-controlagent comprises .01% to .10% of a rare earth.

References Cited UNITED STATES PATENTS 2,683,662 7/1954 Tisdale et al75123 E 3,102,831 9/1963 Tisdale 148-12 3,333,987 8/1967 Schrader et a]148-12.1 3,375,105 3/1968 Lynch 75123 J RICHARD O, DEAN, PrimaryExaminer US. Cl. X.R.

75123 E, 123 H, 123 N, 123 I; 148-l2, 12.3

